blutoonwithcarrotandnail wrote:Stems ending in -IO, -TUDO, -DO and -GO are exceptions and are masculine instead of feminine. Most consonant stems ending with these endings are feminine.

Looking at the exceptions: ORDIN and SERMON

they DO NOT EVEN END in -IO, -TUDO, -DO and -GO

so why is this why they are exceptions?

I assume you are looking at the section of the grammar for stems ending in N. ordo and sermo have stems ending in N (ordin-, sermon-). Nouns with the suffixes -io, -tudo, -do, and -go also have stems ending in N (-ion-, -tudin-, -din-, -gin-). Therefore, sermo and ordo are in the same category as nouns ending in -io, -tudo, -do, and -go (the category of nouns that have stems ending in N). The nouns ending in -io, -tudo, -do, and -go are a subcategory of N-stems that are feminine.

Nouns with the suffixes -io, -tudo, -do, and -go also have stems ending in N (-ion-, -tudin-, -din-, -gin-). Therefore, sermo and ordo are in the same category as nouns

In ORDO you can see the -DO (-DIN-) but i assume that there is
no direct correspondence for SERMO to match any of the following
-ION- -TUDIN- -DIN- -GIN-. It is just in the class of N-Stems because
it is correct?

Nouns with the suffixes -io, -tudo, -do, and -go also have stems ending in N (-ion-, -tudin-, -din-, -gin-). Therefore, sermo and ordo are in the same category as nouns

In ORDO you can see the -DO (-DIN-) but i assume that there isno direct correspondence for SERMO to match any of the following-ION- -TUDIN- -DIN- -GIN-. It is just in the class of N-Stems becauseit is correct?

Thanks.

I think you should have in mind that the N-stems do not necessarily mean N in the nominative. Not every word ending in -do will be a N-stem (or maybe yes, since my Latin knowledge is low, but I wouldn't trust the fact). That's one of the reasons why every substantive in Latin should be learnt with the genitive as well.

ordo, ordinis. sermo, sermonis. homo, hominis. N in the stem, you see it in the genitive. The nominative ending doesn't give any clue about that.

The thing with the endings really doesn't work with other nominative endings such as -us. We have equus, equi (2nd declension, stem in -o), but also manus, manui (4th declension, stem in -u), and tempus, temporis (3rd declension, stem in -r). So I would advice you to look at the genitive first to identify the declension, and not the nominative. It's less prone to confusion this way.